The mental retardation, physical handicaps, and premature aging of Down syndrome (DS) are caused by a triplication of all or parts of human chromosome 21 (CH21). The clarity of this cytogenetic definition belies a complex pathophysiology impacted by an unknown number of genes. The ultimate goal of this proposal is to identify some of these genes and their targets. A novel approach to this problem is proposed, which uses previously generated transgenetic mice and embryonic stem cells containing large genomic regions of CH 21 that have been implicated in the DS phenotype and have been manipulated as yeast artificial chromosomes (YACs). The technique of differential display/reverse transcriptase polymerase chain reaction (DDRTPCR) will be used to detect differential mRNA accumulation from both genes located on the DS region YACs, and on mouse genes that respond to human YAC-encoded gene products. This technique provides a visual representation of differential gene expression, and allows the "capture" aid molecular manipulation of the differences. A series of high-throughput assays are proposed to classify, authenticate, map, and identify resultant cDNAs.